2017A&A...597A.137K

The photospheric radius is one of the fundamental parameters governing the radiative equilibrium of a star. We report new observations of the nearest solar-type stars α Centauri A (G2V) and B (K1V) with the VLTI/PIONIER optical interferometer. The combination of four configurations of the VLTI enable us to measure simultaneously the limb darkened angular diameter θ_LD and the limb darkening parameters of the two solar-type stars in the near-infrared H band (λ=1.65µm). We obtain photospheric angular diameters of θ_LD(A)=8.502±0.038mas (0.43%) and θ_LD(B)=5.999±0.025mas (0.42%), through the adjustment of a power law limb darkening model. We find H band power law exponents of α(A)=0.1404±0.0050 (3.6%) and α(B)=0.1545±0.0044 (2.8%), which closely bracket the observed solar value (α_☉=0.15027). Combined with the parallax π=747.17±0.61mas previously determined, we derive linear radii of R_A=1.2234±0.0053R_☉ (0.43%) and R_B=0.8632±0.0037R_☉ (0.43%). The power law exponents that we derive for the two stars indicate a significantly weaker limb darkening than predicted by both 1D and 3D stellar atmosphere models. As this discrepancy is also observed on the near-infrared limb darkening profile of the Sun, an improvement of the calibration of stellar atmosphere models is clearly needed. The reported PIONIER visibility measurements of α Cen A and B provide a robust basis to validate the future evolutions of these models.